How to reproduce:
1. Open the user's attached "ProBuilderIssue" project
2. Open the Scene "SampleScene"
3. In the Hierarchy select the "Arch" GameObject
4. Open the ProBuilder window (Tools->ProBuilder->ProBuilder window)
5. In the ProBuilder window select "Gen Shadow Obj"

Expected results: No errors are thrown
Actual results: The "ArgumentOutOfRangeException: Array length must match vertex count." error is thrown

Reproducible with: 4.4.0, 5.0.0-pre.10 (2019.4.22f1, 2020.3.1f1, 2021.1.0f1, 2021.2.0a9)
Not reproducible with: 4.3.1 (2018.4.33f1, 2019.4.22f1)

Notes:
- Only occurs on ProBuilder GameObjects created before importing the "Editor example" package

Notes (resolution notes):
- Problem is not specific to the use of Gen Shadow Object and not specific to the import of Editor Example package but is related to domain reload (caused by the package import)